Processes of the above-indicated type are well known in the art, viz. processes in which sulfur dioxide and sulfuric acid formed by oxidation thereof are fixed as sulfite and sulfate in a dry powder which results when a sulfur dioxide-containing flue gas is used as drying gas in a spray drying process in which the liquid which is atomized is a solution of a soluble strong base or a suspension of calcium hydroxide.
Such a process is described e.g. in U.S. Pat. No. 3,932,587. According to this specification the basic liquid is an aqueous solution or suspension containing at the most 40% by weight alkali metal carbonate or hydrogen carbonate, preferably sodium carbonate and/or sodium hydrogen carbonate. Due to the fact that the time is very short in which each atomized droplet of the basic liquid can react with the sulfur dioxide, it has hitherto been regarded as necessary (although the use of a calcium hydroxide suspension as basic liquid has been suggested) that the basic liquid should comprise a solution of a very soluble and very reactive basic substance such as e.g. sodium hydroxide or, as mentioned above, sodium carbonate. Indeed, a certain reaction between the sulfur dioxide and the particles formed by the atomization can take place also after the latter have been dried to a substantially anhydrous powder, but far the dominating part of the absorption reaction takes place while a liquid phase is still present in the atomized particles.
The water soluble substances, e.g. sodium carbonate, coming into consideration in the known processes of this type, are in many places so expensive, and the resulting sulfite- and sulfate-containing powder has so limited market, that the use of said soluble substances mainly comes into consideration in connection with a regeneration stage in which the sulfite- and sulfate-containing powder from the spray absorption-drying process is regenerated to carbonate or hydroxide. Regeneration in connection with the use of alkali metal hydroxide or carbonate as absorbents can, moreover, be necessary because disposal of the resulting sulfite-containing material may cause problems, as there is a risk that the material disposed of may contaminate subsoil water and streams due to the high solubility of the material.
However, such regeneration processes which i.a. are described in the above U.S. specification require an extensive plant and a complex operation, which has impeded the industrial utilization thereof.
Therefore, it is desired to provide a process of the type stated in the first paragraph of the present specification, in which process a basic material, viz. Ca(OH).sub.2, is used, which is so inexpensive and readily available that regeneration of the sulfite- and sulfate-containing product can be dispensed with, and in which the absorbent is utilized effectively and results in a powder which is easy to handle and discharge.
Several processes are known for desulfurization of flue gas, in which processes the flue gas is scrubbed with an aqueous basic liquid, e.g. in a scrubbing tower.
By the processes of this last-mentioned type, the basic liquid is not dried to produce a powder but leaves the reaction zone as a sulfite- and sulfate-containing liquid which is possibly recycled and/or regenerated. Processes of this last-mentioned type are e.g. disclosed in Danish specification No. 123 337, Swedish published patent application No. 371 368, U.S. Pat. No. 3,533,748 and DE-OS Nos. 2 304 496, 2 419 579 and 2 550 488.
The above-mentioned prejudice that calcium hydroxide is not suitable for processes in which the SO.sub.2 absorption and spray drying of the absorbent take place simultaneously has been prevailing in spite of the fact that already in 1960 it was suggested in Czechoslovakian specification No. 96 138 to use a calcium hydroxide suspension as absorbent. In said specification it was suggested to improve the conversion of the calcium hydroxide by partially recycling the resulting powder to the suspension of Ca(OH).sub.2 to be atomized. However, this process has not found any substantial application, probably because the Ca(OH).sub.2 consumption has been unacceptable high in spite of the recycling.
In the method disclosed in said Czechoslovakian specification the fly ash contained in the flue gas is collected before the gas is contacted with the atomized calcium hydroxide suspension. It is not specified which temperature and humidity shall be obtained for the flue gas at the end of the treatment.